This invention relates to an ion implantation beam monitor, in particular for monitoring an ion beam in an ion implanter which implants ions into substrates such as semiconductor wafers.
Semiconductor devices are typically formed from a semiconductor substrate material into which atoms or molecules of selected dopants have been implanted or defused. The dopant particles produce regions in the semiconductor substrate having varying conductivity. By selecting appropriate dopant materials, the majority charge carrier may be locally altered within the substrate.
One preferred technique for adding dopant materials to semiconductor substrates uses ion implantation. This technique minimises the size of the device features created by the dopants within the substrate, reducing the overall size of the semiconductor device itself and increasing operational speed.
The principles of operation of an ion implantation apparatus will be familiar to those skilled in the art. Briefly, a plasma generates positive ions of the selective dopant material in an ion source. The ion source ejects the required positively charged ions, which are accelerated by application of an acceleration potential through a magnetic field. The magnetic field is generated by a mass selection arrangement which deflects the ejected ions around a curved path. The radius of curvature of the flight path of the ions is dependent upon the mass/charge ratio of the individual ions. The exit of the mass selection arrangement has a slit within it so that only ions having a predetermined mass/charge ratio can exit the mass selection arrangement.
Those ions exiting the mass selection arrangement impinge upon a semiconductor substrate to be doped. Typically, this substrate will have previously been patterned with photo resist so that only selected regions are doped. Usually, the ions are decelerated after leaving the mass selection arrangement by a further, adjustable high voltage, to allow the final velocity (and hence, the penetration depth into the wafer) of the ions to be chosen.
Frequently, the cross-sectional area of the ion beam at the substrate is less than that of the substrate. This necessitates scanning of either the substrate relative to a fixed direction ion beam or scanning the ion beam across a fixed substrate. In practice, it is preferable to scan the substrate while maintaining the ion beam in a fixed direction, in a manner described below.
FIGS. 1a and 1b show a typical substrate holder 10 looking along the lines of ions exiting the slit in the mass selection arrangement. The substrate holder 10 comprises a plurality of substrate supports or paddles 20, onto which substrates to be doped may be affixed. The substrates supports 20 are spaced equidistantly from a central hub 22 by a plurality of spokes 24. Located between two of the spokes is a sheet of solid material known as a flag 32. The purpose of the flag is to allow indexing of the substrates, as will be described in more detail below.
The central hub 22 is connected to a drive 26 by a shaft 28. The drive 26, which may for example be an electric motor, drives the shaft 28 such that the hub 22 is caused to move reciprocally in the manner of an inverted pendulum. Referring to FIG. 1a, this motion is indicated by the arrows AAxe2x80x2.
In addition to its reciprocal motion, the hub is also rotated about an axis perpendicular to it, as indicated by the second arrow B in FIG. 1a. Thus, the ion beam, which normally follows a fixed, linear trajectory once it exits the mass selection apparatus, is caused to scan across the plurality of substrates held on the substrate supports 20 by the reciprocating and rotating movement of the substrate holder 10. The motion of the substrate holder relative to the ion beam creates a series of curvilinear xe2x80x9cstripesxe2x80x9d across each substrate.
It will be understood that the reciprocal movement of the substrate holder described herein is but one exemplary method of scanning the ion beam relative to the substrates. For example, the substrate holder may be moved linearly in the vertical plane instead, or in a number of other ways.
In order to ensure that each substrate is doped substantially uniformly across its face, it is useful that the ion beam striking the substrates be monitored. If irregularities or drop-outs in the beam occur (due, for example, to sparking in the high voltage supply that accelerates the ions), then the substrate will be doped non-uniformly. One technique used to carry out such monitoring employs a beam stop, arranged downstream of the ion beam. Such a beam stop is also shown schematically in FIGS. 1a and 1b. 
The beam stop 30 includes a Faraday type current detector 40 which may, for example, be a Faraday cup or bucket. The principles of such a device are well-known. Briefly, the centre of the ion beam is directed toward the Faraday type current detector 40, which absorbs ions from the ion beam impinging upon it. An ion beam current is measured by ancillary circuitry 50, which calculates the current from the accumulated charge in the Faraday type detector 40. As the reciprocal movement of the shaft 28 supporting the hub 22 causes the substrate holder 10 to pass between the ion beam and the beam stop, the ion beam is absorbed by the substrates rather than the Faraday type detector, and the beam stop current reduces.
Monitoring of the ion beam current has, in the past, been carried out simply by observing the beam stop current during those periods when the Faraday type detector 40 is completely uncovered, i.e. when the shaft 28 of the substrate holder 10 has swung away from the beam stop 30, as shown in FIG. 1a. However, in this approach the ion beam drop-outs described above may well be missed, if they occur as the substrate holder moves in front of the Faraday type detector.
It is an object of the present invention to provide an improved ion implantation beam monitor that alleviates the problems with the prior art.
According to a first aspect of the present invention, there is provided an ion implantation apparatus comprising an ion beam source, a substrate holder downstream of the ion beam source, the substrate holder supporting a plurality of radially spaced substrates, scanning means for scanning the substrates relative to the ion beam, a beam stop, downstream of the substrate holder, for capturing ions in the ion beam not striking the substrate holder, and for generating a beam stop current therefrom, and sampling means, for taking a plurality of samples of the beam stop current which is generated as the ion beam passes between the radially spaced substrates, such that the ion beam current may be monitored over a plurality of discrete time periods.
The gap between substrates is thus used to obtain information about the ion beam continuity. By sampling the beam stop current each time the ion beam passes between two adjacent substrates, for example, information can be obtained on the ion dose received by these two substrates. This in turn can assist in identifying a particular substrate which has not received a correct dose, due to drop-outs in the ion beam, for example.
According to a second aspect of the present invention, there is provided an ion implantation apparatus comprising an ion beam source, a substrate holder, downstream of the ion beam source, scanning means for scanning the substrate holder relative to the ion beam at a predetermined scan rate, a beam stop, downstream of the substrate holder, for capturing ions in the ion beam not striking the substrate holder, and for generating a beam stop current therefrom, timing means, for measuring the time difference between a first time, at which the beam stop current first reduces from a maximum value as the substrate holder passes in front of the ion beam, and a second time, at which the beam stop current first reaches a minimum value as the substrate holder passes in front of the ion beam, and output means for outputting the product of the time difference and the predetermined scan rate, the output thereby being representative of the width of the ion beam perpendicular to the direction of travel of said ion beam.
The beam stop current rises and falls as a series of pulses, when the substrates periodically cut the ion beam. Each pulse has a maximum as the ion beam passes between adjacent substrates and a minimum as the ion beam is absorbed by the substrates. Measuring the time between pulses can provide an indication of the ion beam width.
In a third aspect of the present invention, there is provided an ion implantation apparatus comprising an ion beam source, a substrate holder, downstream of the ion beam source, the substrate holder comprising a rotatable wheel including a plurality of substrates supported upon spokes spaced radially around the wheel, a beam stop, downstream of the substrate holder, for capturing ions in the ion beam not striking the substrate holder, and for generating a beam stop current therefrom, scanning means for scanning the substrates relative to the ion beam such that the beam stop current comprises a series of is pulses, each pulse defining an upper beam stop current, where the ion beam passes between the substrates or spokes, and a lower beam stop current, where the ion beam passes across the substrates or spokes, timing means for timing the pulse width of a pulse having a minimum upper beam stop current, and output means for outputting the product of the pulse width and the scan velocity, the output being representative of the height of the ion beam perpendicular to the direction of travel of said ion beam.
The width of the pulse produced as the ion beam passes between two adjacent substrates can thus be used to measure the ion beam height.
The invention also extends to a method of monitoring an ion beam in an ion implantation apparatus comprising generating an ion beam, scanning a substrate holder relative to the ion beam, the substrate holder supporting a plurality of radially spaced substrates, capturing, in a beam stop, ions, in the ion beam not striking the substrate holder, generating a beam stop current from the ions captured in the beam stop, and taking samples of the beam stop current generated as the ion beam passes between the radially spaced substrates, such that the ion beam current may be monitored over a plurality of discrete time periods.
Further details and advantages of the invention are set out in the dependent claims appended hereto.